It is recalled that a Rogowski coil is a toroidal coil without a magnetic core and surrounding the conductor in which the current is to be measured. For an article about measuring current using Rogowski coils, reference can be made to IEE Proceedings Sections A to I, volume 130, part B, No. 5, September 1983, pages 360 to 363.
The electromotive force E induced in such a coil is proportional to the derivative with respect to time of the current I, and which may be expressed by the following equation which is valid to a first approximation: ##EQU1## where: N=number of turns in the coil
.mu..sub.0 =permeability of free space PA1 D=the diameter of the coil former PA1 d=the diameter of the coil wire (including insulation) PA1 R=distance from the center of the conductor to the center of the toroidal section. PA1 l=N.S PA1 and r=4.rho.N(D+d)/d.sub.0.sup.2 PA1 where: PA1 S being the sensitivity of each Rogowski coil with respect to its own phase; and PA1 mij being the sensitivity of Rogowski coil on phase i with respect to the current flowing in adjacent phase j. PA1 m is the sensitivity of each Rogowski coil reative to the current flowing in an adjacent phase; and PA1 S is the sensitivity of each Rogowski coil relative to the current flowing in its own phase.
The term S is referred to as the "sensitivity" of the Rogowski coil.
Finally, it is recalled that the equivalent electrical circuit of a Rogowski coil as seen from the load is constituted by an alternating source E=S .rho.I/.rho.t in series with an inductance l and a resistance r, where:
d.sub.0 =the diameter of the bare wire, and PA2 .rho. is the resistivity of the wire material.
When the current is measured in each of the phases of a three-phase circuit, a Rogowski coil is placed on each phase wire. However, in industrial applications such as medium tension equipment housed in a common cabinet or cubicle, all three coils may be close together. This means that interference from one phase may appear in another phase, and in particular a large current flowing in one of the phases may induce an emf in the coils for the adjacent phases by magnetic coupling.
An object of the invention is to associate a circuit with the measurement system for eliminating the disturbing effects of mutual coupling.
German patent specification DE-A-2 845 155 describes the problem of eliminating mutual induction between coils, but is does not describe means for solving it.